The present invention relates to a.c. to d.c. power converters and more particularly to a three phase a.c. to D.C. converter having reduced source line current harmonics.
Three phase input a.c. to d.c. converters are widely used in both connercial and military applications and particularly in high power applications. In a large number of applications, the d.c. output voltage of the converter is self-regulated through the use of phase angle control in the rectification process or the use of d.c. choppers following rectification.
Generally, all forms of a.c. to d.c. converters act as non-linear loads when operating from an a.c. power system. Non-linear loads generate harmonic currents which are fed back to the a.c. power distribution system where they create voltage drops across source impedances and line inductances which, in turn, produce distortions of the voltage waveforms in the distribution or transmission lines of the network. The voltage distortions which result can also produce system shutdown and possible destruction. It is, therefore, desirable to provide converters which are essentially current harmonic free if voltage transients and waveform distortions are to be controlled and eliminated on power distribution networks.
The bridge converter is generally the basic unit of any three phase a.c. to d.c. converter system. However, all basic converter circuits characteristically produce unacceptably large magnitude current harmonics on the three phase a.c. power lines.
In an attempt to overcome this problem, U.S. Pat. No. 4,143,414 to Brewster, the disclosure of which is incorporated herein by reference, suggests the use of separate signal phase a.c. to d.c. converters for each phase of the three phase input source. Each of the single phase a.c. to d.c. converters employs a full wave bridge rectifier feeding a substantially resistive inverter thereby reducing the source harmonics produced by the a.c. to d.c. converter to the extent that the inverter approximates a resistive load. While this circuit represents an improvement over the prior art converters, it is limited by the compliance range, the ratio of maximum input voltage to minimum voltage, of commercially practical converter circuits.
FIG. 1 is a block diagram of the a.c. to d.c. converter of the Brewster patent. As shown therein, the three phases .0.A, .0.B and .0.C of a three phase voltage source 10 are applied to respective full wave bridge rectifiers 12a, 12b and 12c (each rectifier being referred to generally as rectifier 12) whose outputs are coupled to single phase d.c. to d.c. converters 14a, 14b and 14c (each converter being referred to generally as converter 14), respectively. Each converter 14 draws current from its respective bridge rectifier 12 over an input voltage range determined by the construction of the converter 14 and generates an output power which is added to the outputs of the remaining converters 14.
As noted above, the magnitude of the harmonic currents generated by each converter 14 is determined by the ability of the converter to operate in a manner which approximates a resistive load. The converter 14 will operate in a manner which approximates a resistive load as long as the instantaneous input line current (the current from the respective phase of the three phase source) is proportional to the instantaneous input line voltage. To the extent that the converter 14 can approach this ideal current/voltage relationship, the generated harmonics of the fundamental frequency will approach zero amplitude. The ability of the converter 14 to fulfill this relationship is limited by the input voltage range over which the converter can continue to draw a current proportional to the instantaneous input voltage.
FIGS. 2A and 2B illustrate the relationship between the input voltage and input current waveforms wherein the input line current wavefor is proportional to the input voltage waveform over a conduction angle .theta.. The conduction angle .theta. of the converter 14 is a function of both the compliance range of the converter (i.e., the ratio of maximum input voltage to minimum input voltage over which the converter can properly operate) and the magnitude of operating voltages under which the converter normally operates.
The percentage of harmonics generated by the converter decreases as the conductin angle .theta. increases up to 180.degree.. This relationship is illustrated graphically in FIG. 3 which illustrates the percentage of fifth harmonic currents which will be generated by the converter as a function of the conduction angle .theta.. By way of example, if a three percent harmonic distortion is required, the conduction angle will have to be approximately 146.degree.. If a one percent harmonic distortion is required, the conduction angle will have to be approximately 157.degree.. The ability of the converter 14 to achieve such conduction angles is limited by the relatively limited compliance ranges of practical converters and by the magnitude of operating voltages under which many converters, such as military converters, must operate. This can best be understood by way of example.
A typical converter designed for military use will be required to produce full rated power from approximately 200 volts to 320 volts d.c. instantaneous input. If a three percent distortion is required, the conduction angle must be 146.degree. and the converter 16 must draw current during the time period extending between 17.degree. and 163.degree. of each half cycle of the line voltage. Since the line voltage will have a peak value of no less than 200 volts, the converter 14 must begin drawing current at no more than 200 sine 17.degree.=58 volts. This would necessitate the use of a converter which can operate over the range of 58 volts to 320 volts which is almost a 6 to 1 compliance ratio. Such compliance ratios are difficult to achieve thereby limiting the practical ability of the Brewster design to achieve desired distortion levels.